The origins of the rather dubious rumor can be be traced back to this Entertainment Weekly article, in which Zachary Quinto is interviewed about his new television role.
As you may recall, Quinto played the part of the villain and magical serial killer Sylar on HEROES, and now it's been confirmed that he will be taking on the role of a serial killer once more on the hit television thriller American Horror Story - when his character is revealed to be the newest cold-hearted villain.
It can be dangerous for actors to play similar roles over and over for fear that they might start getting pigeonholed into such roles, effectively limiting their career. In response, Quinto said this:
"It’s like an era of association with certain roles – a specific part like Spock, or a kind of part, like Sylar — is coming to an end... I think it’s safe to say this will be the last serial killer I’ll be playing for the foreseeable future.”
The EW feature writer Jeff Jansen interpreted this to mean that he will nolonger be playing serial killers nor Spock.
However, unless Jansen is relying on unpublished quotes from the above-mentioned interview, it is highly unlikely Quinto meant he would be calling it quits in regards to Star Trek. Yes, he does mention Spock, but specifically emphasizes serial killers as the characters he'd like to stop playing, and Spock is no serial killer, at least as far as we know.
That's not the biggest dent in the rumor, though. We have no announcement to the effect, but I am certain that Quinto would not have been signed onto the Star Trek franchise without a multi-picture deal spanning at least a full trilogy. Paramount would not have been daft enough to sign on a young hot actor like that without such an agreement. He was well-known at the time for his role on HEROES, but he was not a big enough bigshot to negotiate out of a major franchise or hold it hostage for a higher paying contract.
Sure, Quinto could simply not do the films, but Paramount would be unlikely to let him off without whatever penalties were worked into the contract for breaking it.
Finally, it would be a poor career move. Some actors would look at the Star Trek franchise and feel like they don't want to have to deal with all the cons and fans, and etc., but all of the major Star Trek actors have made great careers out of their roles, and Quinto could do a lot worse than a career spent playing Spock, and if he doesn't realize it, I'm sure his agent does.
Unless/until we hear otherwise from the studio or director, I'm calling this rumor unconfirmed and very unlikely.
The new Star Trek film, Star Trek: Into Darkness stars, in addition to Quinto, Chris Pine, Benedict Cumberbatch, Alice Eve, Zoe Saldana, Karl Urban, Simon Peg, Anton Yelchin, and John Cho. The film hits theaters on May 17, 2013.